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RPM gain from lighter axel?

  1. May 13, 2004 #1
    I need to know whether to invest in a new lighter axel for a go kart

    Before I take the plunge, I'd like to get "some" kind of idea of the kinds of gains one can expect from this kind of change.

    The conditions as I know Them.

    the 5Hp engine achieves max torque at 5200-5400 RPM.

    the motor is geared at 6:1 and the gearing from the motor to the rear axel
    is 34/27.. for a combined gear ratio of 4.76:1.

    The current rear axel weighs on the order of 4 Lbs, without tires.
    what kind of RPM gains could one expect from reducing real axel weight
    to 2 lbs?

    Note: the different between first and last is somewhere on the order of 1-2 % of rear axel RPM.
  2. jcsd
  3. May 14, 2004 #2


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    Staff: Mentor

    The weight of the axel will affect your acceleration, but not your top speed. Since a torque is essentially a force, weight (mass) and torque are related by f=ma. Finding out how much is a little difficult because you have to add together the angular acceleration of the rotating parts and the linear acceleration of the car.
  4. May 14, 2004 #3


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    Science Advisor

    In principle, a lighter axle helps acceleration. In practice, however, you won't find a measurable difference. (By axle, I'm assuming you mean the shaft between the final drive and the wheel.) The axle, being far smaller in diameter than the wheels, and considering that moment of inertia is proportional to the square of diameter, has an inertia effect which negligible compared with the inertia effects of the wheels, or even the crankshaft.
  5. May 14, 2004 #4
    Thanks a ton

    You just saved me 675 dollars!
  6. May 15, 2004 #5


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    Science Advisor

    krab is right that you won't notice a difference by changing the axle, but you could with other components. For example, the flywheel and clutch can be important (you still have those on go-karts, right?). You can define an effective mass due to the rotating parts, and you'll find that it is proportional to the square of the gear ratio from the wheels to the part you're talking about. So even if the object's moment of inertia is relatively small, its effect can be magnified a lot.
  7. May 16, 2004 #6
    Dear Steven,

    As Stingray suggests, a lighter flywheel may be worth looking into.

    Years ago, some short track motorcycles reduced the rotating mass of their flywheels for greater acceleration by removing the flywheel's electrical guts.

    Instead of generating its own electricity from the spinning flywheel, the motor ran off a total-loss ignition system. A small battery powered the breaker points and spark plug and easily lasted for a short race.

    In addition to the reduced rotating mass near the rim of the flywheel, the motor was relieved of the far smaller burden of generating the electricity.

    The effect was small, but useful in racing.

    Carl Fogel
  8. May 17, 2004 #7
    Thanks Guys

    Flywheel modifications are not allowed in this stock clas; however, it is possible to work with lighter gears ( reducing rotating mass) on the engine.
    My understanding is that this type of modification would go to acceleration only
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